For a number of desirable applications, local area networks (LANs) interconnect personal computers for transfer of data and files between terminals on the network. For example, electronic mail transfers that share such common resources as memory devices, printers, modems, and other peripherals use a physical interface to transmit data among different terminals.
To proper LAN operations, standards have been developed so that equipment from different vendors easily interconnects and communicates with the LAN. For example, each terminal on a LAN may require that the transceiver function transmit information from the terminal to the network and accommodate receiving information at the terminal from other terminals on the LAN. One example of a LAN standard to accomplish this result is the EIA RS-485 standard, which utilizes what is known as an RS-485 transceiver.
In the RS-485 transceiver, there is a driver circuit that includes one high-side driver circuit and one low-side driver circuit. The high-side and low-side driver circuits are 180.degree. out of phase with each other. According to conventional designs, the RS-485 driver circuit requires one or more large blocking or Schottky diode devices to prevent forward biasing of parasitic PN junctions of certain transistors under certain fault conditions. These large diodes appreciably increase the die area and drive up RS-485 driver circuits costs.
The RS-485 transceiver, to operate according to the RS-485 standard, must have the ability in the RS-485 driver circuit to receive voltages in the range of -7 to +12 volts, and, as a practical matter, withstand voltages between .+-.15 volts as applied to the RS-485 driver circuit's pin connections. The Schottky diodes serve as blocking devices to protect output transistors and other circuitry from the excessive currents that can result from the excessive voltages and also to meet leakage currents specified by the RS-485 standard.
While the Schottky diodes are effective in blocking excessive currents, they have significant limitations. One limitation of using a Schottky diode relates to the significant costs in terms of space and voltage drop that such devices require. Schottky diodes are also output swing limited, especially at cold temperatures, because of the approximate 0.5-volt voltage drop that occurs then. No acceptable alternative to the present Schottky diode configuration exists for either the high-side or low-side driver circuits.
The high-side circuitry of an RS-485 driver circuit suffers from yet a further limitation. This is the problem of assuring that the power output transistor of the high-side circuitry turns off and stays off when the driver circuit output is low. While a Schottky diode works well for this application, again the limitations of space consumption and undesirable voltage load across the diode exist.